Sirius XM Holdings Inc. shares are down 4.1% in Thursday trading after Goldman Sachs analyst Brett Feldman downgraded the stock to sell from neutral. "We believe the stock's material YTD outperformance (+40% vs. +4% for the S&P 500) has been driven primarily by a range of positioning factors including its relationship with its tracking stock declining short interest, and Sirius's high share repurchases relative to its low float," Feldman wrote. "We see limited upside potential to subscriber and financial estimates over the near-to medium-term given the slowing pace of new car sales in the US, a maturing used car subscriber acquisition funnel and limited opportunity to increase pricing in the near-term." He has a $6 price target on the stock. Sirius's stock has gained 43% over the past 12 months, while the S&P 500 has gained 14%.